1. Field of the Invention
The present invention relates to an organic electroluminescent light-emitting element (hereinafter "organic EL element") and more particularly to a light-emitting device using an organic EL element.
2. Description of the Prior Art
A thin-film organic electroluminescent element (thin-film organic EL element) uses light emitted when organic molecules, which have been excited by the recombination of holes (positive vacancies) and electrons, shift to the ground state. Such an organic EL element is sometimes referred to as an "organic field light-emitting diode" or an "organic field LED (light emission diode)".
As FIG. 1 shows, a conventional organic EL element 100 comprises a transparent anode layer 102 which is provided on a transparent substrate 101, a hole injection/carrying layer 103 and an electron transportation and light-emitting layer 104 which are provided sequentially on the transparent anode layer 102. Further, a cathode layer 105 is provided on the electron transportation and light-emitting layer 104. Electric power must be supplied to the organic EL element 100 for emission of light so that the transparent anode layer 102 is positive and the cathode layer 105 is negative. A power source must therefore be provided. Ordinarily, as FIG. 1 shows, batteries 106 or the like are separately provided outside the organic EL element 100 in order to supply power to the organic EL element 100. The batteries 106 may comprise either primary or secondary batteries.
However, as shown in FIG. 1, in a conventional light-emitting apparatus using the above organic EL element, the primary or secondary batteries or the like which constitute the power source must be provided separately. This requires extra space, and is an obstacle to achieving light-weight and small-scale terminal equipment. Furthermore, wiring and connections are needed to provide the batteries separate from the organic EL element, thereby increasing the cost of materials, parts and assembling. Moreover, the electrical resistance of these wires and connections is far from negligible, and accelerates the exhaustion of the batteries.
Japanese Patent Laid-Open No. Sho 59-217991 (JP, A, 59-217991) discloses a light-emitting device wherein a light-emitting element utilizing electroluminescence of inorganic material and a solar cell, which functions as a power source for the light-emitting element, are provided in a multilayered arrangement, and the solar cell is disposed in a single unit manner with an emitting portion. FIGS. 2 and 3 show the light-emitting device disclosed in JP, A, 59-217991. The light-emitting device uses an element comprising an emitting portion 137 and a solar cell 141 which are multilayered in this order in a single unit on a glass substrate 131. The emitting portion 137 comprises an EL emitting layer 134 sandwiched between a pair of dielectric layers 133 and 135. In addition, transparent electrodes 132 and 136 are provided to the outer side of each of the dielectric layers 133 and 135 respectively. The solar cell 141 has a pin-type configuration comprising an n-type amorphous Si (silicon) layer 138, an i-type amorphous Si layer 139 and a p-type amorphous Si layer 140. A metallic electrode 142 is provided on the p-Si layer 140. The transparent electrode 136 of the emitting portion 137 also functions as an electrode for the n-Si layer 138. Here, a thin film, comprising ZnS (zinc sulfide) mixed with a minute quantity of activator substance, is used as the EL emitting layer 134.
However, since the light-emitting device utilizing the element shown in FIG. 2 emits light by means of inorganic electroluminescence, a comparatively high ac (alternating current) voltage must be applied to the EL emitting layer 134. Consequently, as FIG. 3 shows, the light-emitting device further comprises a battery 144 for storing power generated by the solar cell 141, an oscillating/boosting portion 145 for generating alternating current to be applied to the EL emitting layer 134, a sensor 146 for determining daytime or night-time, and a controller 143 for executing control so that, during daytime, power generated by the solar cell 141 is stored in the battery 144 and, during night-time, power from the battery 144 is supplied to the EL emitting layer 134 by means of the oscillating/boosting portion 145.
Since the conventional light-emitting device disclosed in JP, A, 59-217991 (FIGS. 2 and 3) requires, in addition to a solar cell 141 provided in a single unit with-the EL emitting layer 134, a controller 143, a secondary battery 144, an oscillating/boosting portion 145 and a sensor 146, the apparatus is expensive and cannot easily be assembled to small-scale or light-weight. Since electrical energy generated by the solar cell 141 has to be stored in the battery 144 during bright daytime hours, the emission cannot easily be maintained for long periods during daytime. But even when the apparatus is used primarily to emit light at night, the disadvantage remains that, unless the apparatus is positioned in a location which is bright during daytime, the electrical energy required to emit light cannot be stored. A further inconvenience is that, since the only power source is provided by energy generated by a solar cell, there is a limit on the period of time over which the apparatus can be continuously run. The solar cell 141 does not function without injection of light energy. However, in the conventional device, since the emitting portion 137 is disposed at the front of solar cell 141, the emitting portion 137 reflects, disperses and consumes the light intended for the solar cell 141, thereby reducing the amount of light injected to the solar cell 141 and notably damaging the efficiency of the solar cell 141 to generate power. This has the disadvantage that the solar cell 141 does not provide a sufficient power source.
The fundamental reason for the various problems of the conventional light-emitting device shown in FIGS. 2 and 3 is that the voltage source secured in a single unit manner to the emitting portion is a solar cell. In principle, a solar cell is an element which converts light into electric energy, and therefore does not function without light. Furthermore, although a solar cell may be capable of solar-electric power generation, it has no ability to store electrical energy and consequently requires a battery and the function as a power source is obtained only by combining the solar cell and the battery. Moreover, since this power source is obtained by means of light energy conversion, a light sensor and a controller thereof are also required. Such a complicated configuration makes this conventional light-emitting device extremely unsuitable for small-scale or light-weight manufacture, such as for portable equipment and the like.
Furthermore, the light-emitting device disclosed in JP, A, 59-217991 comprises a common electrode which functions as one electrode of the emitting portion 137 and as the electrode of the n-Si layer side of the solar cell 141. However, since light must be injected to the solar cell, the electrode used is restricted to a transparent electrode such as the transparent electrode 136 shown in FIG. 2. This transparent electrode must comprise electrode material having high electric conductivity. Furthermore, since emitting portion 137 is an inorganic EL element using electroluminescence of inorganic material as explained above, the emission principle of this emitting portion 137 is essentially different from the emission principle of an organic EL element. In other words, in the inorganic EL element, a light-emitting layer is sandwiched between a pair of dielectric layers, electrons are accelerated and collide by means of a large electric field applied from the electrodes via each dielectric layer to the luminescence center of the light-emitting layer, and electroluminescence thereby generated is used. The inorganic EL element therefore requires a large electric field. Further, the inorganic EL element is ordinarily driven by an ac power.
By contrast, as described above, the organic EL element uses light emitted by the energy difference created when organic molecules, which have been excited by the recombination of a hole and an electron, shift to the ground state. Therefore, the organic EL element is able to emit light with a low-voltage direct current. Emission using electroluminescence differs completely in principle between an inorganic EL element and an organic EL element.
In a conventional light-emitting device using an organic EL element, batteries or the like must be provided separately, hindering small-scale and light-weight manufacture of the apparatus. In addition, the electrical resistance of connection wires and the like accelerates exhaustion of the batteries. Furthermore, in a conventional light-emitting device wherein an inorganic EL element is multilayered in a single body manner to the solar cell, a battery and an oscillating/boosting portion are additionally required. Moreover, the inorganic EL element weakens the light injected to the solar cell, noticeably damaging the power generation efficiency.
If an organic EL element is provided in a multilayered arrangement with a solar cell or a chemical battery, the electrodes of the organic EL device must be not only conductive but must also be efficient emitters of electrons and positive holes. In such an arrangement the battery electrode and the organic EL element cannot easily be formed from the same material.